Ball valves are a very popular choice for a variety of applications in which fluid control is needed—both to shut off the flow and to control the amount of flow. At its most basic level, a ball valve is simply a spherical plug (a ball) mounted to rotate inside a valve body or housing, where a cylindrical flow path has been provided in the valve body and through the ball. The valve body may be mounted between two pipes so that the valve may control the flow of fluid through the pipes. When the flow path through the ball is positioned completely transverse to the flow path through the valve body, there will be no flow. When the flow paths through the body and ball are aligned longitudinally with each other, there will be maximum flow. When the valve is between full-open and full-closed, the flow will be throttled because only a portion of the flow path through the ball will be aligned with the flow path through the valve body. Generally, the ball rotates under the control of a shaft that extends from a connection on the ball inside the valve body to the outside of the valve body. The shaft can have a handle mounted on it for manual operation, or it may be driven by an actuator.
When the valve is fully closed, it is desirable that no fluid be allowed to pass from the upstream portion of the valve body to the downstream portion, either through the ball or around the ball. As a result, it is common to use various techniques to create a seal between the ball and the valve body. The seal between the ball and the valve body is typically produced by placing a ring-shaped flexible seat around the periphery of the flow path against the upstream face of the ball. The inner edge of the seat is held in place against the ball and flexes with irregularities in the shape of the ball, when the ball is rotated, to provide a consistent and adequate seal. The seat can be held in place, so that it does not fall out of the valve or get swept into the fluid flow, by pressing a retaining ring against it along its outer edge.
Various forces combine to resist rotation of a ball valve, and thus require additional strength from a human operator or an actuator. Frictional forces—both static and dynamic—between various moving parts in a valve are a major contributor to the resistance of rotation. One of the major frictional forces occurs between the flexible seat and the ball face. The seat must be positioned sufficiently close to the ball face so that the two stay in sealed contact even when the seat and the ball are at their greatest distance relative to each other due to dimensional variations that cannot be eliminated from the valve (such as eccentricities in the rotational radius of the ball). In addition, pressure of the process fluid on the upstream side of the seat can increase or decrease the force between the seat and the ball face, and thereby affect the frictional forces that resist rotation of the valve. A valve needs to be designed so that it is positively sealed under the worst-case for tolerances and low upstream pressure (even as the sealing surface of the seat wears down over time). As a consequence, when the upstream fluid is at high pressure and the seal tolerances are at their tightest, the friction between the sealing surface and the ball is at a maximum. These design considerations should be taken into account while minimizing the rotational force required to operate the valve, because increased force requires stronger and more expensive actuators, especially for large valves.
Therefore, it is desirable to have a seat for a ball valve that provides a good seal without exerting excessive friction on the ball segment, that maintains an adequate sealing force across a range of seat wear, and that is compatible with a variety of fluids across a wide range of temperatures and pressures.